1. Field of the Invention
The present invention relates to a crane apparatus which is used, for example, as a transfer crane of port cargo handling equipment.
2. Description of the Related Art
A conventional transfer crane has a gate-shaped body frame, which can travel by a plurality of traveling wheels. On the top of the body frame, a trolley can move transversely. On the trolley, a rotationally drivable takeup drum is borne. A hoisting accessory is attached to the lower end of a plurality of load suspending wire ropes wound off from the takeup drum. Thus, as the trolley moves on the body frame, a suspended load held by the hoisting accessory can be traversed. When the takeup drum is rotationally driven at a predetermined stop position of the trolley to take up or pay out the plural wire ropes, the suspended load can be moved upwards or downwards.
With the above-described conventional crane apparatus, rails are provided on transverse girders placed on the body frame, and the trolley is supported on the rails. In consideration of the weight of the rails, the weight of the trolley, or the weight of the suspended load imposed on the hoisting accessory, not only the transverse girders and the body frame, but also the entire crane apparatus must have great rigidity, inducing a large size and a heavy weight. When the trolley is moved laterally and stopped at a predetermined position, with the suspended load being held by the hoisting accessory, sway occurs in the hoisting accessory and the suspended load owing to an inertial force working in the lateral direction. This sway does not settle quickly. To prevent sway of the hoisting accessory and the suspended load when the trolley stops, the trolley has to be moved at a slow speed. Since a long time is required for work, the work efficiency is low.
Under these circumstances, the applicant filed an application entitled xe2x80x9cCrane Systemxe2x80x9d as International Application PCT/JP98/05448. In the xe2x80x9cCrane Systemxe2x80x9d, wire ropes are passed over two sheaves provided at a distance in a right-and-left direction on a hoisting accessory; an end of one of the wire ropes is passed over a first drum, and the other end of the one wire rope is passed over a second drum; an end of the other wire rope is passed over the first drum, and the other end of the other wire rope is passed over the second drum; the drums are rotated by a hoisting/lowering motor via a rotation control device in a direction in which the wire ropes are simultaneously taken up or paid out, to enable the hoisting accessory to be hoisted or lowered; and the drums are rotated by a traversing motor via the rotation control device in a direction in which one of the wire ropes is taken up, while the other wire rope is paid out, to enable the hoisting accessory to be traversed. Thus, there is no need to provide rails and a trolley on the body frame. Since the weight on the body frame is reduced, the crane system can be made compact and light-weight. Besides, the hoisting accessory is supported by the wire ropes at the two separated points. Hence, sway of the hoisting accessory and the suspended load due to their inertial force is suppressed, and the work efficiency is increased. Furthermore, the weight of the hoisting accessory and the suspended load is shared between the two drums, so that the hoisting/lowering motor and the traversing motor can be reduced in capacity.
When the hoisting accessory is to be traversed in the foregoing conventional xe2x80x9ccrane systemxe2x80x9d, the drums are rotated by the traversing motor in the direction in which to pay out one of the wire ropes and take up the other wire rope. In this case, when the hoisting accessory (suspended load) is situated in a central portion in the right-and-left direction of the body frame, the lengths of the respective wire ropes from the hoisting accessory to right-hand and left-hand sheaves are the same. Thus, tensions acting on the respective wire ropes, namely, reaction forces acting on the respective drums, are the same. When the hoisting accessory (suspended load) is moved leftward or rightward relative to the body frame, the lengths of the respective wire ropes change. Thus, a difference in tension occurs between the respective wire ropes, and the reaction forces acting on the respective drums vary. As a result, the traversing motor requires a varying driving force according to the traversing position of the hoisting accessory. Consequently, the capacity of the traversing motor cannot be made sufficiently low. During traversal of the hoisting accessory (suspended load), for example, its movement from left to right in the body frame, rotation of the respective drums by the traversing motor alone results in a downwardly arcuate moving path of the hoisting accessory, because the takeup amount and the payout amount of the right and left wire ropes are the same. To avoid this situation, the hoisting/lowering motor is driven by an amount corresponding to the downward motion of the hoisting accessory to lift the hoisting accessory, thereby moving the hoisting accessory horizontally. As noted from this action, both the traversing motor and the hoisting/lowering motor have to be driven during traversal of the hoisting accessory. Thus, the capacity of each motor cannot be made sufficiently low, either.
With the above-mentioned xe2x80x9ccrane systemxe2x80x9d, each drum is shaped like a cone so as to minimize a moment due to the difference in tension between the respective wire ropes. That is, the drum diameter on the side wound with the wire rope having the higher tension is made smaller, while the drum diameter on the side wound with the wire rope having the lower tension is made larger. By so doing, the imbalance between the moments of the forces imposed on the respective drums is corrected. Even if the hoisting accessory leans leftward or rightward, torques acting on the respective drums are canceled out. When each drum is conical, the relationship between the takeup amount and payout amount of the right and left wire ropes on the drum is constant, so that the tension ratio of the wire ropes is also constant at a predetermined position of the hoisting accessory. However, the tension ratio (tension difference) between the right and left wire ropes differs according to the ascending or descending position of the hoisting accessory. Merely forming the drum in a conical shape does not make it possible to cancel out torques acting on the drums in all areas in which the hoisting accessory moves.
The present invention has been accomplished to solve the above-described problems. It is an object of this invention to provide a crane apparatus designed to cancel out torques acting on a pair of drums in all areas in which a hoisting accessory moves, so that the capacity of a drive motor can be reduced.
A crane apparatus according to the present invention comprises:
a body frame;
a pair of drums rotatably mounted on the body frame;
a hoisting accessory capable of attaching and detaching a load thereto and therefrom;
a pair of wire ropes having an end taken up by the drums, and having the other end connected to the hoisting accessory;
a traversing motor mounted on the body frame;
hoisting accessory traversing means for traversing the hoisting accessory by rotating the drums by driving of the traversing motor to take up or pay out the wire ropes;
speed change means for changing rotational speed of at least one of the drums; and
control means for controlling the speed change means in a gear changing manner so that forces imposed on the drums by the wire ropes will be balanced.
According to this constitution, torques acting on the pair of drums are canceled out in all areas in which the hoisting accessory moves. As a result, the capacity of the traversing motor can be reduced. Furthermore, the hoisting accessory horizontally moves during traversal. Thus, the hoisting accessory can be traversed simply by driving the traversing motor, and the capacity of the hoisting/lowering motor can be reduced.
In the crane apparatus, it is desirable that when the driving output of the traversing motor is designated as P, the tensions of the respective wire ropes as T1, and T2, and the velocities of the respective wire ropes as V1 and V2, the control means sets the change gear ratio xcex1=T1/T2 of the speed change means such that the following equation will hold:
P=(xcex1T1xe2x88x92T2)(V1/xcex1xe2x88x92V2)/2=0
In the crane apparatus, it is also desirable that when the angles of the respective wire ropes to a horizontal line are designated as xcex81 and xcex82, and the weight of the hoisting accessory having the load suspended therefrom is designated as m, the tensions T1 and T2 of the respective wire ropes are calculated from the following equations:
T1=mgxc2x7cos xcex82/sin(xcex81+xcex82)
T2=mgxc2x7cos xcex81/sin(xcex81+xcex82)
In the crane apparatus, the speed change means may be composed of first speed change means provided on a first driving force transmission path from the traversing motor to one of the drums, and second speed change means provided on a second driving force transmission path from the traversing motor to the other drum, and the change gear ratio xcex1 may be achieved by the change gear ratio xcex11 of the first speed change means and the change gear ratio xcex12 of the second speed change means. By so doing, the change gear ratio of one speed change means can be set to be low, so that the apparatus can be downsized.
In the crane apparatus, the speed change means may be a continuously variable transmission. By so doing, a gear changing action during traversal of the hoisting accessory can be performed smoothly, whereby the hoisting accessory can be traversed stably.
In the crane apparatus, the speed change means may be provided on each of a first driving force transmission path from the traversing motor to one of the drums, and a second driving force transmission path from the traversing motor to the other drum. By so doing, a gear change range by each speed change means can be set to be narrow, so that the apparatus can be made compact and light-weight.
The crane apparatus may further include a hoisting/lowering motor mounted on the body frame, and hoisting accessory hoisting/lowering means for hoisting and lowering the hoisting accessory by rotating the drums by the driving of the hoisting/lowering motor to take up or pay out the wire ropes. Thus, the traversal and the hoisting or lowering of the hoisting accessory can be performed simultaneously to increase the work efficiency.